Your laptop doesn’t really know how much life there is in your battery

It just guesses it by maintaining an internal “idea” of the battery life (which is usually wrong), a bit like you guessing how much petrol is in your car based on when you fill it up at the petrol station. So there could be another couple of hours in your battery, especially if you had a crappy old battery in there before. To calibrate it again, follow this stupid long boring procedure: